1. Field of the Invention
The present invention relates to a device for transferring data from a source to a destination while comparing the data at the source with those at the destination, and more specifically to a data transfer accounting device and method for use in a switched multi-megabit data service (SMDS).
2. Description of the Related Art
With higher performance information processing devices for workstations, personal computers, etc., the users demand to perform quick operations in transmitting data between the information processing devices and between LANs accommodating the information processing devices. For example, the SMDS functions as a high-speed and broadband connection-less data service for realizing the inter-LAN connection. The SMDS is a connectionless data switch service based on the transfer speed of 1.5 Mbps or 45 Mbps.
When the SMDS is provided through an ATM network, a predetermined ATM switch is provide with an SMDS processing device (SMDS message handler), and a permanent virtual circuit or a permanent virtual channel (PVC) connects SMDS subscribers to each other. The connectionless data output from the SMDS subscriber is transferred to the SMDS processing device and performs a routing process, etc. through the processing device.
However, since the above described PVC is a path set in the ATM network, the connectionless data is converted (disassembled) into an ATM cell format before being input to the ATM switch. The cell is a 53-byte unit comprising a 48-byte payload and a 5-byte header.
FIG. 1 is a conventional data format showing the conversion between variable-length data and fixed-length data in the SMDS.
A Protocol data unit (L3-PDU) corresponds to the above described connectionless data, and comprises a header storing a data destination address DA and a source address SA, etc., and a payload. The layer-2 protocol data unit (L2-PDU) corresponds to the above described cell and comprises a beginning of a cell (BOM), an intermediate cell (COM), and an end of a cell (EOM). The BOM stores the source address SA of the connectionless data and the destination address DA. The number of the COMs to be generated depends on the data length of the connectionless data. To simplify the explanation, a fixed-length cell corresponds to the L2-PDU while the variable-length data frame corresponds to the L3-PDU in the following descriptions.
When the data output from the SMDS subscriber and converted into a cell format is input to the SMDS processing device through the PVC set in the ATM network, it is assembled in the format of the L2-PDU or data in a higher-order layer as necessary to make various checks by analyzing the routing information, etc. Then, it is disassembled into cells and routed according to the analysis information.
The accounting process of the SMDS is described in the Technical Reference TR-TSV-000775 issued by Bell Communication Research in the U.S. When data is transferred from the SMDS subscriber A to the SMDS subscriber B in the accounting process, the accounting process is performed on the data receiving side, that is, by the SMDS processing device accommodating the SMDS subscriber B. Then, the following data is collected in the accounting process:
destination address DA (address of SMDS subscriber B); PA1 source address SA (address of SMDS subscriber A) carrier (information identifying the communications service provider of the path from the SMDS processing device accommodating the SMDS subscriber A to the SMDS processing device accommodating the SMDS subscriber B; PA1 SNI address (subscriber network interface address of the SMDS subscriber B); PA1 segment count (number of L2-PDUs); and PA1 packet count (number of L3-PDUs).
The SMDS processing device comprises a line unit for transmitting data in a cell format and a common unit for monitoring and controlling the entire process of the SMDS processing device. An accounting unit is provided in the common unit. When the line unit receives cell data, it generates a cell frame signal having a pulse synchronous with the cell data and a cell enable signal indicating whether or not the cell data is valid. These signals are transferred to the common unit in parallel with the cell data at a predetermined timing in synchronism with the cell data.
The accounting unit recognizes the segmentation of the cell data, and uses the cell frame signal as a refreshing signal for the accounting data accumulating DRAM. If a cell enable signal indicates a valid cell in the accounting process, the information stored in the valid cell (collected information, etc.) is retrieved.
In the ATM network used for the SMDS, data can be transferred at a desired speed in a way that either valid cells or invalid cells are transferred at predetermined intervals. Therefore, the line unit in the SMDS processing device outputs a cell frame signal having pulses at predetermined intervals when receiving cells (valid and invalid cells). If the period of the pulses of the cell frame is disturbed or the pulses cannot be generated due to a fault, etc. in the line unit, then the accounting unit may not be able to perform the accounting process.
Therefore, the SMDS processing device monitors the pulse period and disconnection state of the cell frame signal. If it detects a fault, an appropriate action should be taken against the fault. However, the conventional SMDS processing device may be provided with the function of monitoring the disconnection of the cell frame signal, but is not provided with the function of monitoring the pulse period of the cell frame signal.
As described above, when the cell enable signal indicates a valid state, the information stored in the cell is collected as accounting data. Accordingly, if the cell enable signal indicates a valid state even though an invalid cell or a cell containing a transmission error has been input, then the communications fee may be charged to the account of a wrong subscriber.
Therefore, it is required to take action against the case where an abnormal condition occurs in the cell enable signal to improve the reliability of the accounting process. However, the conventional SMDS processing devices have no such function.
The problem related to the reliability of the accounting process should be carefully studied by the users as well as the communication services providers. The conventional SMDS accounting processes are performed on the basis of normal cell frame signals and cell enable signals, and are not provided with sufficient check and fail-safe capabilities for use in abnormal conditions.
The above listed problems are not only caused in the SMDS but are common to all systems for performing accounting processes using transfer data timing information or the transfer data valid/invalid information.
Recently, in specific areas and companies, a local area network (LAN) designed by inter-connecting a plurality of terminal units such as work stations, etc. is commonly used to transmit information.
With the development of information technology, LANs are used more and more frequently. Furthermore, by interconnecting the LANs located at various positions, it is possible to design a system for wide transmission of information.
The SMDS divides a variable-length packet (layer 3 protocol data unit, or L3-PDU) into a plurality of fixed-length cells (layer 2 protocol data unit) according to the SMDS interface protocol (SIP), and information is transferred in a fixed-length cell.
That is, in the SMDS, data is transmitted cell by cell in the L2-PDU format in the switching system in which cells are switched.
The fixed-length cell has the data format shown in FIG. 2.
That is, as shown in FIG. 2, an L2-PDU comprises 16 bits.times.27 of data (54 octets). An area 2100a having the data length of 5 bits stores message ID (MID) information. An area 2100b having the data length of 8 bits stores SNI-ID information. An area 2100c having the data length of 8 octets stores DA information. An area 2100d having the data length of 8 octets stores SA information. An area 2100e is a trailer unit.
A message ID (MID), which is the information stored in the area 2100a, indicates the type of message transmitted between subscribers. A destination address DA indicates an address on the receiving side, while a source address SA indicates an address on the sending side. A subscriber network interface identifier (SNI-ID) indicates the type of subscriber network interface (address).
In the L2-PDU, the header unit stores the segment type (ST) information (not shown in the drawings) indicating the portion in the undivided L3-PDU. The information can be a beginning of a message (BOM) indicating a cell containing the leading portion of the original L3-PDU, an end of a message (EOM) indicating a cell containing the last portion, and a continuation of a message (COM) indicating a cell containing an intermediate portion other than the BOM or EOM, and a single segment message (SSM), complete in a single cell.
In the above described SMDS switch system, an accounting process is performed as shown by TR-TSV-00775 of the technical reference of Bell Communications Research in the U.S. to determine the network fee to be charged to each subscriber's account in the SMDS network.
That is, the accounting process in the SMDS switch system is performed by the accounting system as shown in FIG. 3.
The accounting system shown in FIG. 3 is represented in the block diagram relating to the accounting system operated by the LP5813 shown in FIG. 5.
The accounting system shown in FIG. 3 comprises on the SMDS switch network side a data generation unit 3101; a data aggregating unit 3102; a data formatting unit 3103; a data transmitting unit 3104; and a RAO(Revenue Accounting Office) processing unit 3105 on the revenue accounting office side.
The data generating unit 3101 determines whether or not the SMDS accounting data is required as usage information of a packet (L3-PDU) in the SMDS, and generates the accounting data as necessary, by configuring the SMDS accounting data generating device.
The data aggregating unit 3102 receives data from the data generating unit 3101 and aggregates the number of packets and segments transferred between a source subscriber and a destination subscriber at predetermined intervals.
The data formatting unit 3103 formats the accounting information as the usage information of the SMDS switch network, the audit information, and time change information in a predetermined format before processing the accounting information by an accounting information managing unit 3105.
The data transfer unit 3104 transfers the formatted usage information, audit information, and time change information from the data formatting unit 3103 to the accounting information managing unit 3105.
The accounting information managing unit 3105 receives data from the data transfer unit 3104 and manages the accounting information.
With the above described configuration, the accounting data generated by the data generating unit 3101, data aggregating unit 3102, and data formatting unit 3103, is transferred by the data transfer unit 3104 to the accounting information managing unit 3105. The accounting information managing unit 3105 receives the data from the data transfer unit 3104 to manage the accounting information and perform a requested accounting process for each subscriber.
The data generating unit 3101 generates the accounting data for each object L3-PDU. The accounting data comprises 6 parameters, that is, a DA, an SA, an SNI address, a condition code, a segment count, and a packet count. It further comprises transmission line carrier information, etc.
The condition code indicates the transfer state of information. Practically, it indicates whether or not an error exists, and also indicates an error type. An SNI address indicates the address of a subscriber network interface. A segment count indicates the number of divided L2-PDUs. A packet count indicates the number of L3-PDUs.
Since a large volume of data should be handled to process the accounting data by the data generating unit 3101, these parameters are normally compressed in the generating process of the accounting data if the object parameter type can be limited.
For example, the DA can be limited relatively easily. That is, the above described technical reference TR-TSV-000772 describes "up to 16 types of individual addresses and up to 48 types of group addresses can be defined for one SNI". Accordingly, there are up to 64 types of individual addresses and group addresses. Thus, each SNI can be limited to up to 64 types of address information.
Similarly, since the SNI address can be determined by a DA, it can also be limited in type. The above described technical reference defines only some types of condition codes. Therefore, it can also be limited. Likewise, the carrier information can be limited. However, the SA cannot be easily limited.
That is, as indicated by the above described technical reference TR-TSV-000772, the SA screening process is performed on the receiving side. In the SA screening process, an SA screening table is provided on the receiving side, and the SA of a received L3-PDU is compared with the SA entered in the table to determine whether or not the received L3-PDU can be accepted.
There are two types of algorithms to determine whether to receive the received L3-PDU or not: the algorithm for receiving when the SA of the received L3-PDU is included in the SA registered in the above described table; and the algorithm for not receiving when the SA of the received L3-PDU is included in the SA registered in the above described table.
The above described technical reference TR-TSV-000772 also defines the maximum number of the SA screening processes. Either algorithm for the SA screening can be selected as the network when subscribing the network. When the former algorithm is selected, up to 128 SA can be accepted and the SA parameter can be limited, thereby possibly compressing the SA. When the latter algorithm is selected, the number of SAs to be accepted cannot be specified. Therefore, the SA cannot be limited, thereby hardly compressing the SA.
Using a common telephone unit, the user talks with specific persons in most cases, and rarely receives calls from non-specific persons. Therefore, in the SMDS switch, the accounting data can be collected when the SAs of the specific persons are prepared even if all the possible SA bit patterns are not prepared.
An SMDS accounting data generating device shown in FIG. 4 can be used as having the functions of the data generating unit 3101 and data aggregating unit 3102 shown in FIG. 3.
In FIG. 4, a DA compressing unit 4111 receives a fixed-length cell as an L2-PDU, divided from the L3-PDU as shown in FIG. 2. From the header information of the L2-PDU, the DA compressing unit 4111 reads the DA information, SA information, carrier information, SNI address information, MID information, and data length information, compresses the DA information, and outputs the result as the DA compressed information RDA. The input data is delayed and output as being unchanged.
An ST identifying unit 4112 identifies a segment type based on the header information of the L2-PDU from the DA compressing unit 4111.
The ST (segment type) indicates the type of the L2-PDU forming the message when a message is transmitted between subscriber terminal units.
An accumulating RAM 4113 accumulates the DA compressed information RDA from the DA compressing unit 4111, SA information, and carrier information according to the SNI address information.
An SA compressing unit 4114 receives the SA information and carrier information from the accumulating RAM 4113 and compresses the SA information, and outputs the SA compressed information RSA and carrier information.
An (RSA+carrier) compressing unit 4115 receives the information RSA from the SA compressing unit 4114 and the carrier information, and compresses the combination of various types of the information into a single piece of information.
An L2-PDU adding unit 4116 counts the number of the L2-PDUs corresponding to the DA information, SA information, carrier information, MID information, and SNI-ID information.
An L3-PDU adding unit 4117 counts the number of the L3-PDUs comprising the L2-PDUs corresponding to the DA information, SA information, carrier information, MID information, and SNI-ID information.
Accounting RAM 4118a and 4118b store the SA information, carrier information, the count value information from the L2-PDU adding unit 4116, and the count value information from the L3-PDU adding unit 4117.
The addresses of the accounting RAM 4118a and 4118b form a 2-dimensional matrix using the two address information bits of the DA compressed address and (RSA+carrier) compressed address.
With this configuration, the SMDS accounting data generating device shown in FIG. 4 generates DA compressed information RDA. The accumulating RAM 4113 stores the DA compressed information RDA, SA information, and carrier information according to the SNI address information and MID address information.
The SA compressing unit 4114 compresses the SA information, and the (RSA+carrier) compressing unit 4115 compresses the SA compressed information RSA generated by compressing the SA information and the carrier information, and generates the information RSAC.
Then, the accounting RAM 4118a and 4118b generates the SMDS accounting data by writing the SA information, carrier information, segment count information from the L2-PDU adding unit 4116, and the packet count information from the L3-PDU adding unit 4117 to the 2-dimensional matrix, according to the DA compressed information RDA and the information RSAC generated by compressing the RSA and carrier information.
However, in the accounting data generating system of the SMDS accounting data generating device shown in FIG. 4, when the numbers of SNIs, individual addresses, group addresses, and receivable messages are increased, it is also required to increase the number of RAM correspondingly.
That is, when the number of receivable SAs is to be increased for the accounting RAM 4118a and 4118b having the address configuration of the above described 2-dimensional matrix, a sufficient RAM area is required corresponding to the number of the DAs with the increasing number of (RSA+carrier) patterns. In the present hardware technology, it is impossible to provide an accounting RAM area large enough to support the number of received messages, for practical use.
In consideration of the present hardware technology, the compressed DA can be limited, but the (RSA+carrier) can be limited only to 2.sup.8 =256 types when the LSI forming the compressing unit and the capacity of the RAM are taken into account. If it is exceeded, there is no area to write data in the 2-dimensional matrix, thereby all excess accounting data is discarded.
For example, when the number of SNIs is 32, the number of individual addresses is 8, the number of group addresses is 8, and the number of receivable messages is 256, then the RAM area required for the messages is calculated by the following equation (1). EQU 32.times.(8+8).times.256=131072 (messages) (1)
When the number of SNIs is 32, the number of individual addresses is 8, the number of group addresses is 8, and the number of receivable messages is 1024, then the RAM area required for the messages is calculated by the following equation (2). EQU 32.times.(8+8).times.1024=524288 (messages) (2)
When the number of SNIs is 64, the number of individual addresses is 16, the number of group addresses is 48, and the number of receivable messages is 2048, then the RAM area required for the messages is calculated by the following equation (3). EQU 64.times.(16+48).times.2048=8388608 (messages) (3)
The broadband connectionless data service requires efficient processing of subscriber information when processing connectionless data for a large number of subscribers.
FIG. 5 shows a common configuration of an SMDS system.
The SMDS is a high-speed connectionless data service for transferring data after dividing a variable-length message in a level-3 protocol data unit (L3-PDU) into a fixed-length data in a packet, which is similar to a cell in an asynchronous transfer mode (ATM) in which data are transferred in a level-2 protocol data unit (L2-PDU). In addition to user information, which is actually transferred to the L3-PDU, a source address SA and a destination address DA are added to the L3-PDU. The same message identifier, that is, identification information, is added to the L2-PDUs forming the same L3-PDU.
The information from a subscriber is processed through a subscriber network interface (SNI). The subscriber is provided with a DS1 subscriber line 5801 having the transmission speed of, for example, 1.5 Mbps, and a DS3 subscriber line 5803 having the transmission speed of 45 Mbps, as physical transmission lines. A subscriber terminal unit divides the L3-PDU in the SMDS into one or more L2-PDU, converts the L2-PDU into the data format for the DS1 subscriber line 5801 or DS3 subscriber line 5803, and then outputs the resultant data to each subscriber line.
The DS1 subscriber line 5801 is terminated by a DS1 terminating unit 5802, while the DS3 subscriber line 5803 is terminated by a DS3 terminating unit 5804. The L2-PDUs are received from these terminating units.
The L2-PDU from the subscriber is assigned a semi-fixed VCI (virtual channel identifier) and VPI (virtual path identifier) corresponding to the SMDS. Each of the DS1 terminating unit 5802 and DS3 terminating unit 5804 extracts the L2-PDU provided with the semi-fixed VCI and VPI, replaces the VCI+VPI of the L2-PDU with the semi-fixed VCI+VPI corresponding to each terminating unit, and then outputs the L2-PDU to a switch unit 5808. The switch unit 5808 is probably an ATM switch. In this case, the L2-PDU is first converted into an ATM cell and output to the switch unit 5808. Since the L2-PDU and ATM cell are fixed-length packets containing data of equal length, the ATM switch and SMDS have an affinity for each other.
The L2-PDU (ATM cell) output from the DS1 terminating unit 5802 or DS3 terminating unit 5804 is input to the switch unit 5808 through the multiplexer/demultiplexer 5805, 5806, or 5807. The switch unit 5808 transfers the L2-PDU assigned a semi-fixed VCI+VPI corresponding to each of the DS1 terminating unit 5802 or DS3 terminating unit 5804 to an SMDS line part (LP) 5813.
The LP 5813 is connected to the switch unit 5808 through the highway 5815 having the transmission speed of 622 Mbps. The LP 5813 can process up to 64 subscriber interfaces (SNI) accommodated by a station containing the LP 5813. The number of the LPs 5813 corresponding to the scale of the station are connected to the switch unit 5808 through the highway 5815.
The LP 5813 can recognize from which DS1 terminating unit 5802 or DS3 terminating unit 5804 the received L2-PDU has been input by determining the semi-fixed VCI+CPI assigned to the L2-PDU. The LP 5813 extracts the source address SA from the message (L3-PDU) transferred using a plurality of L2-PDUs, and calculates the reduced destination address DA corresponding to the extracted address SA. The LP 5813 extracts the destination address DA from the message. If the destination address DA is addressed to a destination within the station, then the LP 5813 replaces the VPI+VCI of each L2-PDU of the message stored after being divided with the semi-fixed VPI+VCI of the DS1 terminating unit 5802 or DS3 terminating unit 5804 in the station corresponding to the destination address DA. On the other hand, if the destination address DA is addressed to a destination outside the station, then the LP 5813 calculates the LP of the destination station and replaces the VPI+VCI of each L2-PDU of the message stored after being divided with the reduced source address and the VPI+VCI corresponding to the LP 5813 of the destination station. Then, the LP 5813 outputs each of the L2-PDUs to the switch unit 5808 through the interface 5814 and highway 5815.
Each switch module (not shown in the drawings) in the switch unit 5808 quickly switches an input L2-PDU. In this case, if the L2-PDU input from the LP 5813 is addressed to a destination within the station, it is transferred to a subscriber in the station through the switch unit 5808, multiplexer/demultiplexer 5805, 5806, or 5807, and through the DS1 terminating unit 5802 or DS3 terminating unit 5804, and the DS1 subscriber line 5801 or DS3 subscriber line 5803. If the L2-PDU is addressed to a destination outside the station, then it is transferred to the LP 5813 of the destination station through a multiplexer/demultiplexer 5810, terminating circuit (FINF) 5811, and an inter-station relay line 5812.
The LP 5813 at the destination station realizes that the message is addressed to a destination in the station by determining the destination address DA of the message transferred using a plurality of received L2-PDUs. Then, the LP 5813 replaces the VPI+VCI of each L2-PDU storing the message with the semi-fixed VPI+VCI of the DS1 terminating unit 5802 and DS3 terminating unit 5804 in the station corresponding to the destination address DA, and outputs each L2-PDU to a switch unit. Each of these L2-PDUs is transferred to the subscriber in the station through the switch unit 5808, DS1 terminating unit 5802, or DS3 terminating unit 5804, and through the DS1 subscriber line 5801 or DS3 subscriber line 5803.
As described above, the DS1 terminating unit 5802 for terminating the DS1 terminating unit 5802 or the DS3 terminating unit 5804 for terminating the DS3 subscriber line 5803 as shown in FIG. 5 does not individually process subscriber information, but the LP 5813 processes all subscriber information. Thus, the hardware costs can be kept down for the entire switching unit, and the maintenance can easily be made. A control device 5809 should access the LP 5813 through a control line when processing a call, but does not have to individually access each of the DS1 terminating units 5802 and DS3 terminating units 5804. Therefore, the subscriber information can be efficiently processed.
In the broadband connectionless data service such as the SMDS, an accounting process should be performed for each subscriber
In the accounting process, subscriber information generated as a result of the communications set by each subscriber should be collected for each subscriber.
In this case, the accounting process should be performed on the communications data for which network services are provided. Therefore, the accounting process is performed at the destination station. Practically, the process is performed by the LP 5813 shown in FIG. 5.
As a method often cited as an accounting process, the LP 5813 collects and accumulates subscriber information (accounting data) required for the accounting process for each of the source addresses SA stored in the L3-PDUs addressed to the receiving station. The source address SA stored by the L3-PDU has the information of, for example, 64 bits, and the number of the source addresses SA is 2.sup.64. Therefore, it is practically impossible for the LP 5813 to accumulate the accounting data relating to such a large number of source addresses SA.
Thus, the Applicant has proposed a method of collecting and accumulating accounting data about each reduced source address after degenerating a source address SA extracted from received L3-PDUs into some thousands of reduced source addresses each of which is represented by data of ten or more bits.
Thus, the accounting process can be realized by the LP 5813 using common memory elements.
The following sequence is used as a practical process sequence of the accounting process performed by the LP 5813. That is, accounting data is collected for each of the reduced source addresses SA, for example, every minute. The accounting data collected for 15 minutes are provided for the control device 5809 (refer to FIG. 5).
FIG. 6 shows the conventional accounting process for realizing the above described accounting process sequence. The configuration shown in FIG. 6 is provided in the LP 5813 shown in FIG. 5.
An accounting data collecting unit 6901 collects accounting data DATA about each reduced source address RDSCA after degenerating a source address SA extracted from received L3-PDUs into some thousands of reduced source addresses RDSCA, each of which is represented by data of ten or more bits. The accounting data collecting unit 6901 writes alternately every minute on collecting phases 6902 (phases 0 and 1), that is, memory elements, the collected accounting data DATA with the reduced source address RDSCA.
Concurrently, an MPU 6905 transfers the accounting data DATA collected for one minute in the collecting phase 6902 on which the accounting data collecting unit 6901 is not writing data, to a reading phase 6903 (bold line A in FIG. 6), which is a memory element for storing the accounting data DATA for 15 minutes, through a bus 6908 using a RAM 6907 connected through the bus 6908 as a work memory, according to a control program stored in a ROM 6906 connected through the bus 6908. When the accounting data DATA for 15 minutes is stored on a reading phase 6903, the MPU 6905 collectively transfers all contents written on the reading phase 6903 to a notifying phase 6904, that is, a memory element equal in storage capacity to the reading phase 6903 (bold line B in FIG. 6).
Finally, the control device 5809 retrieves the contents written on the notifying phase 6904 (shown in FIG. 6) in the LP 5813.
When the MPU 6905 transfers the accounting data DATA collected for 1 minute in a collecting phase 6902 to the reading phase 6903, for storing the accounting data DATA for 15 minutes in the sequence of the conventional accounting process shown in FIG. 6, the MPU 6905 needs to retrieve from the reading phase 6903 the accounting data DATA written in the collecting phase 6902 to calculate the total data calculated for 15 minutes. If the accounting data DATA on the collecting phase 6902 is detected on the reading phase 6903, then the MPU 6905 merges the accounting data DATA in the collecting phase 6902 into the accounting data DATA detected on the reading phase 6903. If the accounting data DATA in the collecting phase 6902 is not detected on the reading phase 6903, then the MPU 6905 adds the accounting data DATA in the collecting phase 6902 into the new accounting data area on the reading phase 6903.
Thus, the data transfer of the accounting data DATA from the collecting phase 6902 to the reading phase 6903 is not a simple transfer process but a process requiring retrieval of the accounting data DATA in the reading phase 6903. In this case, the maximum number of messages to generate the accounting data DATA collected in the collecting phase 6902 matches the maximum number of the above described reduced source addresses RDSCA. Therefore, the accounting data DATA containing some thousands of messages should be retrieved in the reading phase 6903 within the one minute in which the processing phases are switched in the collecting phase 6902.
However, in a software process performed by the MPU 6905, it may be impossible to perform the entire process. In the worst case, all accounting data DATA cannot be transferred from the collecting phase 6902 to the reading phase 6903, by possibly generating communications in which no accounting process can be performed.
Practically, the maximum number of the accounting data DATA collected in the collecting phase 6902 is n. Then, a data area for 4 addresses is retrieved for 1 set of accounting data DATA with 32 bits set for 1 address. The amount of data for one set of the accounting data DATA can include 4 or more addresses. An average of 20 clock pulses that is, 40.times.10.sup.-9 .times.20 seconds, is required by the MPU 6905 to retrieve data at one address in the reading phase 6903 with the process clock pulse width of the MPU 6905 set to 40.times.10.sup.-9 seconds.
At this time, if up to (n.times.14) sets of different accounting data DATA are stored on the reading phase 6903 for the past 14 minutes, and if the maximum n sets of the accounting data DATA are collected in the last minute of the 15 minutes, then the time calculated by the following equation (4) is required to retrieve in 4-address units the accounting data DATA for the past 14 minutes on the reading phase 6903 for one set of the accounting data DATA on the collecting phase 6902. EQU (40.times.10.sup.-9 .times.20) seconds.times.4 addresses.times.(n.times.14) sets (4)
Therefore, the time calculated by the following equation (5) is required to retrieve in 4-address units the accounting data DATA for the past 14 minutes on the reading phase 6903 for each of the n sets of the accounting data DATA in the collecting phase 6902. EQU {(40.times.10.sup.-9 .times.20) seconds.times.4 addresses.times.(n.times.14) sets}.times.n sets (5)
Since the upper time limit is the minute in which the processing phases are switched on the collecting phase 6902, the maximum number of sets n of the accounting data DATA collected in the collecting phase 6902 is expressed by the following expression (6). EQU N.apprxeq.1157 sets (6)
That is, n.apprxeq.1157 or more sets of the accounting data DATA cannot be collected even with the MPU 6905 having the process bit width of 32 bits.
This type of problem is not limited to the accounting data processing device, but is commonly relating to the device for transferring data from a source to a destination while comparing the data at the source with the data at the destination.